A look at some of the most common puzzlers faced by Scala beginners.

1. What Can Scala Puzzlers
Teach Us?
Daniel C. Sobral

2. Disclaimer #1: "Puzzlers"
● Java Puzzlers (the book/presentations):
○ Weird behavior in Java that is difficult to predict or
explain
● This presentation:
○ Weird behavior in Scala that confounds beginners
■ (and sometimes experienced programmers)
○ FAQs, not Fun

3. I can't speak for any of the people who
contributed to create and turn Scala into what it
is. Furthermore, if you learn anything from this
presentation, learn that a perfect understanding
of Scala is an unlikely achievement...
Disclaimer #2: don't trust me.

4. Symbols, operators and punctuation
● What does it mean?
● Where can I find information about it?
● WTF?

5. The symbol puzzler
Source: Stack Overflow

6. The Origin of the Symbols
● Punctuation elements of the language
○ @ # ( ) [ ] { } , . ; : ` ' "
● Keywords, reserved symbols and XML
○ <- => <: >: <% // /* */ _* <? <!
● Normal definitions from libraries
○ <:< =:= ☆ ★ η :/
● Underscore
○ All of the above!

7. Symbols may be part of the language or come
from external libraries; it can be difficult to tell
where they begin and end; they are difficult to
search for on the Internet, non-mnemonic and
unpronounceable.

8. Then why?
def f(xs: Array[Int], a: Int, b: Int) =
for (i <- xs.indices)
xs(i) = a * i + b

9. So that libraries can extend the
language seamlessly
def f(xs: M[T], a: T, b: T) =
for (i <- xs.indices)
xs(i) = a * i + b
For M and T library-types that support these
operations

10. Which still doesn't really justify unicode...

11. Advice regarding Symbols
● Learn the language:
○ What symbols are "built-in"
○ The syntax rules
● If exploring code using unknown libraries,
○ Use an IDE to explore it
harsh, indeed...

12. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = {
new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] {
def id[A] = ☆(_ η)
def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g
}
}
Let's try to figure this one out

13. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = {
new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] {
def id[A] = ☆(_ η)
def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g
}
}
Split Scala Syntax from Identifiers

14. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = {
new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] {
def id[A] = ☆(_ η)
def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g
}
}
Identify what we are defining

15. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = {
new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] {
def id[A] = ☆(_ η)
def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g
}
}
Where we are using our definitions

16. implicit def KleisliCategory[M[_] : Monad]: Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] = {
new Category[({type λ[α, β] = Kleisli[M, α, β]})#λ] {
def id[A] = ☆(_ η)
def compose[X, Y, Z](f: Kleisli[M, Y, Z], g: Kleisli[M, X, Y]) = f <=< g
}
}
What's left are outside identifiers

17. The Underscore
● What does it mean?
● Why does it work here but not here?
● WTF?
● It's everywhere! It's the borg operator!

18. One underscore puzzler
Source: Stack overflow

19. The meanings of underscore
Source: myself, by way of Stack Overflow

20. The underscore is Scala's wildcard; it means
"something" or "anything" and even "nothing".
However, it's semantics can be widely different,
even in similar contexts.

21. Why?
● ???
● Picking a different symbol for each case
would stretch ASCII
● Picking keywords is not in the spirit of Scala
● They are somewhat intuitive
○ except when they bite you

22. Advice regarding underscore
● Intuition does go a long way
● But, again, learn the language
● And, in particular, know the difference
between these:
○ f _
○ f(_, y)
○ f(x + _, y)

23. Eta, partial function application and
placeholders
● f _
○ an eta expansion: turn method f into an anonymous function.
● f(_, y)
○ a partial function application: creates an anonymous function from
method f, whose parameters are the ones wholly replaced by
underscores.
● f(x + _, y)
○ a placeholder: creates an anonymous function from the expression "x
+ _", whose parameters are the underscores (and that function gets
passed to f).

24. _ always picks the tightest non-degenerate
scope it can
Seth Tisue

25. Erasure
● Where are my types?
● How do I get them back?
● WTF?

26. Some things that don't work
trait X {
def overload(f: Int => Int)
def overload(f: String => String)
}

27. Some things that don't work
def f[T](list: List[T]) = list match {
case _: List[Int] => "Ints"
case _: List[String] => "Strings"
case _ => "???"
}

28. Some things that don't work
class C[T] {
def f(obj: Any) = obj match {
case _: T => "Ours"
case _ => "Not Ours"
}
}

29. Why?
● JVM does not support "type parameters"
● If Scala added metadata to "eliminate"
erasure, the interaction gap between it and
Java would be much greater
○ Though things like implicits and default parameters
already do it, to some extent
○ At least, implicits and defaults are easy to avoid

30. Some things that don't work
trait X {
def overload(f: Function1[Int, Int])
def overload(f: Function1[String,
String])
}

31. Erasure at work
trait X {
def overload(f: Function1)
def overload(f: Function1)
}

32. Erasure at work
def f(list: List) = list match {
case _: List => "Ints"
case _: List => "Strings"
case _ => "???"
}

33. Erasure at work
class C {
def f(obj: Any) = obj match {
case _: Any => "Ours"
case _ => "Not Ours"
}
}

34. A partial solution
class C[T : ClassTag] {
def f(obj: Any) = obj match {
case _: T => "Ours"
case _ => "Not Ours"
}
} though it will still ignore T's type parameters

35. Initialization Order
trait A {
val a: Int
val b = a * 2
}
class B extends A {
val a = 5
println(b)
}
new B

36. All val’s are initialized in the order they
are found.

37. Initialization Order
trait A {
val foo: Int
val bar = 10
println("In A: foo: " + foo + ", bar: " + bar)
}
class B extends A {
val foo: Int = 25
println("In B: foo: " + foo + ", bar: " + bar)
}
class C extends B {
override val bar = 99
println("In C: foo: " + foo + ", bar: " + bar)
}
new C
Source: Paul Phillips scala-faq by way of scala puzzlers

38. Initialization Order
In A: foo: 0, bar: 0
In B: foo: 25, bar: 0
In C: foo: 25, bar: 99

39. Initialization Order
trait A {
val foo: Int
val bar = 10
println("In A: foo: " + foo + ", bar: " + bar)
}
class B extends A {
val foo: Int = 25
println("In B: foo: " + foo + ", bar: " + bar)
}
class C extends B {
override val bar = 99
println("In C: foo: " + foo + ", bar: " + bar)
}
new C

40. A val is initialized only once.

41. Initialization Order
trait A { val x = 5 }
trait B {
val x: Int
println(x * 2)
}
trait C extends A with B
trait D extends B
class E extends D with C { println(x * 2) }
class F extends C with D { println(x * 2) }
new E
new F

42. Initialization Order
0
10
10
10

43. Initialization Order
trait A { val x = 5 }
trait B {
val x: Int
println(x * 2)
}
trait C extends A with B
trait D extends B
class E extends B with D with A with B with C { println(x * 2) }
class F extends A with B with C with B with D { println(x * 2) }
new E
new F

44. Trait linearization...

45. Advice regarding initialization order
● Learn the _________

46. Other things to look out for
● Pattern matching on val's and for's
● All sorts of things implicit
● Syntactic sugars, auto-tupling and the like
● The return keyword

47. So, what did I learn?
It seems to me that there's a wide range of
Scala features that depend on the user having
deep knowledge of the language when
something goes wrong. "What's going on?"
seems to be much more common than "How do
I do this?"

48. As for real Puzzlers...
● Site:
○ http://scalapuzzlers.com/
● Book:
○ http://www.artima.com/shop/scala_puzzlers